In the manufacture of semiconductor devices and the like, a stepping or step-and-scan projection aligner has been used in which a pattern of a reticle (photomask) is transferred onto each shot region on a wafer coated with a photoresist through a projection optical system.
The resolution of the projection optical system provided in the projection aligner increases as the exposure wavelength used becomes shorter and the numerical aperture of the projection optical system becomes greater. Therefore, the exposure wavelength which is a wavelength of radiation used in the projection aligner has been reduced along with scaling down of integrated circuits year by year, and the numerical aperture of the projection optical system has also been increased.
In addition to the resolution, the depth of focus is also important for exposure. The resolution R and the depth of focus δ are respectively shown by the following formulas.R=k1·λ/NA  (i)δ=k2·λ/NA2  (ii)wherein λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. When obtaining the same resolution R, a larger depth of focus δ is obtained by using radiation with a shorter wavelength.
A photoresist film is formed on the surface of an exposure target wafer, and the pattern is transferred to the photoresist film. In a conventional projection aligner, the space in which the wafer is placed is filled with air or nitrogen. When the space between the wafer and the lens of the projection aligner is filled with a medium having a refractive index of n, the resolution R and the depth of focus δ are shown by the following formulas.R=k1·(λ/n)/NA  (iii)δ=k2·nλ/NA2  (iv)
For example, when using water as the above medium in the ArF process, since water has a refractive index n for light with a wavelength of 193 nm of 1.44, the resolution R is 69.4% (R=k1·(λ/1.44)/NA) and the depth of focus is 144% (δ=k2·1.44λ/NA2) of the values during exposure using air or nitrogen as the medium.
Such a projection exposure method in which the wavelength of exposure radiation is reduced to transfer a more minute pattern is called a immersion lithography. The immersion lithography is considered to be an essential technology for lithography with reduced dimensions, particularly for lithography with dimensions of several ten nanometers. A projection aligner used for the method is known (see Patent Document 1).
In the immersion lithography, a photoresist film applied and formed on a wafer and the lens of the projection aligner respectively come into contact with a medium with a refractive index n. If water is used as a medium, for example, water may permeate the photoresist film and reduces the photoresist resolution. In addition, the components forming the photoresist may be eluted into the water and may pollute the surface of the lens of the projection aligner.
A method of forming a top coat on the photoresist film in order to shut-off the photoresist film from a medium such as water has been proposed. Such a top coat must possess sufficient transmittance to the wavelength of radiation, must be formed on the photoresist film without causing intermixing with the photoresist film, must maintain a stable film which is not eluted into the medium during immersion lithography, and must easily dissolve in an alkaline solution used as a developer.
Moreover, a resist designed for use in a normal dry environment must be used as a liquid immersion resist as is. In order to provide such a resist film, a top coat for liquid immersion that can protect the resist film from an immersion liquid without degrading its performance originally designed for use under dry conditions is necessary.
[Patent Document 1] JP-A-11-176727